The art of lithographic printing is based on the immiscibility of ink and water. A lithographic printing plate is composed of ink receptive regions, commonly referred to as the “image area,” and hydrophilic regions. When the surface of the printing plate is moistened with water and printing ink is applied, the hydrophilic regions retain the water and repel the printing ink, and the image area accepts the printing ink and repels the water. The printing ink retained on the image area may then be transferred to the surface of a material upon which the image is to be reproduced. Typically, the ink is first transferred to an intermediate blanket, which in turn transfers the ink to the desired surface.
One method for forming or generating an oleophilic image area on a substrate is by coating the substrate with a radiation-sensitive layer, and then exposing a portion of the layer to infrared (IR), ultraviolet (UV) or visible radiation. The unexposed portion of the coated substrate (negative-working plates) or the exposed portion of the coated substrate (positive-working plates) may then be removed by developing the coated substrate in a suitable liquid developer to form the oleophilic image area.
An example of a radiation sensitive composition suitable for use in printing plates is reported in U.S. Pat. No. 5,340,699 to Haley et al. Haley et al. reports a radiation sensitive composition that includes a resole resin, a novolak resin, a latent Bronsted acid and an infrared absorber. Upon imagewise exposure to UV or IR radiation in the presence of the acid, the resole resin and novolak resin react to form a matrix. After exposure, exposed portions of the radiation sensitive composition may be either more or less developable in a developer liquid than unexposed portions of the radiation sensitive composition. Although plates formed by the method reported in Haley et al. exhibit high image resolution, durability, and long press life, the plates require a costly radiation exposure step.
An alternative method for forming an oleophilic image area on a substrate is to imagewise ink-jet an oleophilic composition onto the substrate. Ink-jet application of an oleophilic image area may avoid the costly radiation exposure step and the development step because an oleophilic image area is formed directly on a substrate by ink-jet application and subsequent drying. Many attempts have been made to develop an oleophilic composition suitable for ink-jet application onto a substrate to form a durable, image area that adheres to a substrate. For example, EP 1 211 063, EP 1 157 827, U.S. Pat. No. 6,359,056, EP 1 157 826, EP 1 157 828 and EP 1 157 825 report the formation of image areas via ink-jet application of various oleophilic compositions. Unfortunately, oleophilic images areas formed by direct imagewise ink-jet application tend to lack suitable adhesion to substrates, image resolution, durability, and/or press life.
Thus, it would be advantageous to form a durable, high resolution image area for a printing plate from materials used in conventional radiation sensitive coatings, while avoiding costly radiation exposure steps by utilizing direct imagewise ink-jetting techniques.